Which POD do you have? One of the bean PODs or an XT Live? The reason I ask is that the PODs actually have more and better models and effects than the SpiderIII heads do anyway.

You know, the BOSS Gt-8 is an awesome modeler as well; many say it sounds better than the PODs. IIRC you can also set the GT-8 up in such a way that it can switch channels on your amp at the same time that it switches effects presets and/or models within the effects unit. In other words, no tapdance! Hit one switch and you get your effects AND your channel switching. I'm not sure if the XTLive does that and since I haven't played a GT-8 I don't know exactly how you set that up, but I know a lot of guys at the Jackson guitar forums love that unit for that ability, plus its sounds.

An alternative is that you use the POD as your preamp and effects unit and use both amps' power amp ins. again, the POD actually has better sounds than the Spider anyway, and it also has pretty damn good 5150 and Recto models. The Marshall models on the POD are not as good as the 5150 and Recto models.

Now it's NOT going to sound AS good as the real 5150, but just for shits try using that model into the 5150's power stage and see if you like it. Because if you can eliminate tapdancing it might be worth a slight compromise. And if it doesn't sound right you don't have to keep it, after all.